It has been previously shown that the addition of halogens (e.g. fluorine or the like) to the surface of intermetallic-based titanium alloys, such as TiAl, has a profound effect on the oxidation resistance of such alloys [Schütze et al. 1997 and Kumagai et al. 1996]. The presence of halogens is believed to provide a mechanism of increased aluminum flux to the surface, thereby allowing the alloy to form a protective Al2O3 scale and prevent further oxidation of the alloy. A method for introducing halogens, such as ion implantation, into cast TiAl components has proved to be difficult and expensive [Donchev et al. 2010]. Additionally, results indicate that nearly 70% of the surface halogen is lost after one minute of exposure at 900° C. with a 99% loss after 10 minutes of exposure [Donchev et al. 2003].
In order to introduce a halogen into the bulk of a component and create a reservoir of halogen for improved oxidation resistance, a powder metallurgy approach was previously suggested [Paul et al. 2010] where a halogen-containing gas is used during atomization to alloy the halogen with molten droplets and then solidify into powder therefore containing the halogen in the powder and the bulk component when consolidated. However, there are drawbacks to this single-step atomizing approach, first is that the rate of reaction is extremely rapid at molten titanium temperatures and it is difficult to predict and control the amount of halogen in the resulting solidified powders. Another potential limitation is that if the halogen is in solution in the molten state and in the subsequently solidified powders, no passivation (protective) coating on the powder exists and handling highly reactive powders (i.e. Ti-based) with no passivation can be difficult and dangerous [Jacobson et al. 1964]. Furthermore, the use of a halogen at such high temperatures may have unwanted chemical reactions. For example, if sufficient halogen, such as fluorine, is present near the surface of a molten droplet, titanium fluorides such as TiF3 and TiF4 may form and subsequently volatize into toxic gases [K. Katamura et al. 1986]. Additionally, if halogen containing gasses such as NF3 or SF6 are used, the unwanted constituents of N or S may also be introduced into the molten alloy droplets, resulting in degraded properties.